In recent decades, advances in the understanding of semiconductor materials have made possible the fabrication of thermoelectric heat pumps based on the Peltier effect that have efficiencies adequate for applications in many specialized situations that are characterized by (1) moderate to low temperature differences, (2) low heat flux levels and (3) requirements of low weight, small size and high reliability.
However, wider use of thermoelectrics in cooling and heating applications has been limited by the fact that at increased heat flux levels, vapor compression heat pumps have better efficiency and lower cost than the thermoelectric modules that are now available moreover.
There is a widely held belief within the industry that little further improvement is possible with currently availabe thermoelectric materials. This point is made in a typical manner by H. J. Goldsmid in his book, THERMOELECTRIC REFRIGERATION (Plenum Press, 1964) on page 210: "The precise level of cooling power below which the thermoelectric method is preferable depends on a number of factors, but for most purposes it is about ten watts and will remain in this region until there is a substantial improvement in the figure of merit of the thermoelectric elements."
Such an understanding of thermoelectricity ignores what is certainly one of its most valuable attributes. This is the fact that the thermoelectric element, when operating as a heat pump, is unlike a vapor compression system in that it does not experience a profound drop in the "reduced coefficient of performance" (actual efficiency as compared to the ideal Carnot efficiency) when it operates across a very small temperature difference.
When a vapor compression pump is operating across a large temperature difference, most of the input energy is absorbed in useful compression, with friction and hydrodynamic losses being only a small fraction of the overall energy picture. However, when the temperature difference between the heat source and the heat sink approaches zero, little useful compression occurs, while the friction and hydrodynamic losses are undiminished. As a result, the reduced coefficient of performance becomes vanishingly small.
In contrast, an optimized thermoelectric element will exhibit a monotonic increase in the reduced coefficient of performance as the temperature difference approaches zero. Moreover, the quantity of semiconductor material in an optimized element of fixed heat transport capacity also approaches zero as the temperature difference approaches zero.
It therefore follows that for all heat flux levels that fall between temperature differences less than some specific value, thermoelectric heat pumps are cheaper to operate and less expensive to build than vapor compression devices.
To the knowledge of this inventor, this fact is nowhere exploited or explicitly stated.
The reason why this potential has not been recognized or exploited involves several factors that have worked together to prevent the optimization of thermoelectric heat pumps for low temperature differences.
A major barrier has been the more common use of thermoelectric elements as power generators in addition to their heat pump applications. When used as a power source based on the Seebeck effect a thermoelectric element must be optimized for operation across a large temperature difference, because the Second Law of Thermodynamics limits accessible power to very small precentages of heat flux when the temperature difference is small.
Although the situation is reversed for a heat pump--where the Second Law allows increasingly efficient heat transport as the temperature difference is decreased--the driving force in thermoelectric research has remained the power generating potential. As a result, the hardware available for refrigeration apparatus has often been limited by the need to play a dual role.
Another problem is that the common manner in which thermoelectric modules are assembled employs a structure which can not be optimized for very low temperature differences. Specifically, the need to separate the heat source and heat sink from the thermoelectric element by means of an electrical insulator introduces two unreduceable heat insulating elements into the thermal circuit, which have a profound effect on module performance when operating across small temperature differences. Although these electrical insulators are not theoretically required for the design of thermoelectric devices, they are necessary in this standard structure.
A third barrier to understanding has been the preponderance of applications which are characterized by large temperature differences. That the applications characterized by low temperature differences are few in number falsely suggests that this an area of minor significance. In fact some of these applications have very great economic importance. For example, distillation of water and other liquids by either boiling-condensation or the freeze-melt cycle is most efficient when driven by a heat pump operating across a very small temperature difference.